Supramolecular Architectures from Ammonium-Crown Ether Inclusion

DOI: 10.1021/cg100292u

Supramolecular Architectures from Ammonium-Crown Ether Inclusion Complexes in Polyoxometalate Association: Synthesis, Structure, and Spectroscopy

2010, Vol. 10 3149–3163

Tanmay Chatterjee, Monima Sarma, and Samar K. Das* School of Chemistry, University of Hyderabad, P. O. Central University, Hyderabad 500 046, Andhra Pradesh, India Received March 4, 2010; Revised Manuscript Received May 21, 2010

ABSTRACT: The tetrahedral (Td) molecular cation ammonium ion (NH4þ) gets incorporated with the smaller-cavity crown ethers through three Nþ-H 3 3 3 O hydrogen bonding interactions, while the fourth N-H bond is projected outward from the crown ether cavity. A hydrogen bonding acceptor, such as a polyoxometalate anion, if placed in suitable position in the crystal lattice, can interact with the ammonium-crown ether supramolecular complex involving the fourth N-H bond donation. This article describes supramolecular association of ammonium-crown ether host-guest complexes with polyoxometalate anions in five crystalline solids, formulated as [NH4(B18C6)][Bu4N][Mo6O19] (1), [NH4(B18C6)]4[SiW12O40] 3 2CH3CN (2), [NH4(DB18C6)]2[Mo6O19] 3 4H2O (3), [NH4(DC18C6)]2[Mo6O19] (4), and [NH4(DB30C10)]2[Mo6O19] 3 CH3COOH (5), where B18C6 = benzo-18-crown-6, DB18C6 = dibenzo-18-crown-6, DC18C6 = dicyclohexyl-18-crown-6, and DB30C10 = dibenzo-30-crown-10. Single crystal X-ray structural investigations on these solids confer P1 space symmetry (No. 2) for the compounds 1-2; C2/c space symmetry (No. 15) for the compound 3; P21/n space symmetry (No. 14) for the compound 4, and C2/m (No. 12) space symmetry for the compound 5. Careful examination of the supramolecular interactions between the different molecular fragments show direct contact between the ammonium cation and the polyoxometalate anion (Nþ-H 3 3 3 O) in the crystal structure of the compounds 1, 2, and 4. Lattice water molecules have taken the role of hydrogen bonding acceptor (Nþ-H 3 3 3 O) in the crystal structure of compound 3, while the larger and flexible crown ether DB30C10 has wrapped around the ammonium cation in the case of compound 5. Thus, there is no direct supramolecular interaction between the ammonium cation and the polyoxometalate anion (Nþ-H 3 3 3 O) in the crystal structures of compounds 3 and 5, where the ammonium-crown ether cations are associated with the polyoxometalate anions only via C-H 3 3 3 O hydrogen bonding interactions. Detailed spectroscopic (IR, 1H NMR, and UV-visible) analyses have been included in comprehending the supramolecular association between the inclusion complexes and polyoxometalate anions. The X-ray powder diffraction analyses have been performed to scrutinize the phase purity of the solids.

*To whom correspondence should be addressed. E-mail: skdsc@uohyd. ernet.in. Phone: þ91 40 2301 1007. Fax: þ91 40 2301 2460.

(NH4þ) is a tetrahedral guest cation, that is generally included in the smaller crown ethers (e.g., 18-crown-6) via three Nþ-H 3 3 3 O supramolecular interactions and the fourth N-H bond is projected outward from the crown ether cavity and can be involved in Nþ-H 3 3 3 A hydrogen bonding interactions (A=acceptor) with an acceptor atom suitably placed in the crystal lattice (Scheme 1). Structurally flexible larger crown ethers might result in alteration of the theme by wrapping around the ion thereby making it inert toward the aforesaid acceptor atom (in terms of supramolecular interactions). Polyoxometalates (POMs) or metal-oxide based cluster anions encompassing electronic and structural diversites5 comprise negatively charged oxygen atoms on their surfaces that can participate in hydrogen bonding interactions with a donor atom. As a result, an ammonium-crown ether supramolecular complex cation can be associated with a POM anion both by Coulombic as well as supramolecular Nþ-H 3 3 3 O hydrogen bonding interactions after placing the latter at a suitable acceptor site in the crystal as mentioned above. A little ambiguity might be there for structural prediction because the POM anion can use either terminal or bridging O atom as the H-bond accepting site (Scheme 1). However, these cluster anions are symmetrical, and upon operation of the symmetry elements on these supramolecular interactions a supramolecular network is expected to be generated. Moreover, the C-H 3 3 3 O interactions between the crown ether and the POM anion would also play an important role toward the crystal packing. Therefore, an attempt to perform engineering with the POM anions

r 2010 American Chemical Society

Published on Web 06/14/2010

Introduction Since Pedersen’s serendipitous discovery of dibenzo-18crown-6,1 the macrocyclic crown poly-ethers have been extensively used in the areas of supramolecular chemistry and/or crystal engineering as hosts for variety of cationic species.2 Numerous crown ether related structural entries in the Cambridge Structural Database (CSD) illustrate the matter. These structurally flexible macrocycles encapsulate small guest molecular cations of the type DHnmþ, for example, hydroxonium ion (H3Oþ), ammonium ion (NH4þ), etc., through supramolecular D-H 3 3 3 O hydrogen-bonding interactions (D = donor) and thus act as good stabilizing ligands for these guests. The structure of the resulting supramolecular cationic species is controlled by the cavity size of the crown ether hosts. Larger and structurally more elastic crown poly-ethers bind smaller cations (cation size considerably smaller than the crown ether cavity) either by wrapping around the ion or by intriguing more than one ion in their cavities. A recent perspective “crown ethers as stabilizing ligands for oxonium ions” covers most of the issues where oxonium ions (H3Oþ, H5O2þ, H7O3þ, etc.) have been integrated with the crown ether hosts.3 There are some reports in which a simple ammonium ion (NH4þ) has been shown to be included in the crown ether cavity.4 Simple oxonium ion (H3Oþ) is almost a planar guest cation which is incorporated with the macrocycles through three Oþ-H 3 3 3 O interactions, whereas ammonium ion

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Scheme 1. Schematic Representation Showing Diverse Interactions between the Ammonium Crown-Ether Cation and Segment of a POM Anion As Hydrogen Bonding Acceptor

associated with ammonium-crown ether supramolecular counter cations would be an interesting task. At this point, it is worthy to mention that several multidimensional assemblies formed by the POM macroanions as supramolecular bricks and molecular interactions (covalent or weak) as supramolecular cement have been well-documented in literature.6-8 Earlier we,9 Wang,10 and other research groups11 have described the inclusion chemistry of POM associated crown ether inclusion complexes. Here, in this article, we have described five POM associated ammonium-crown ether supramolecular complexes (compounds 1-5) using four different crown ethers, namely, benzo-18-crown-6 (B18C6), dibenzo-18crown-6 (DB18C6), dicyclohexyl-18-crown-6 (DC18C6), and dibenzo-30-crown-10 (DB30C10) and two different POM cluster anions ([Mo6O19]2- and [SiW12O40]4-). All these macrocyclic poly-ethers (crown ethers) are distinct from each other in terms of their symmetry as well as their cavity size. For example, dibenzo-30-crown-10 has a large cavity and is the most flexible crown-ether among all the five used. The hexamolybdate anion [Mo6O19]2- is an octahedral symmetrical (Oh) iso-polyanion and the silicotungstate anion [SiW12O40]4- is a tetrahedral symmetrical (Td) hetero-polyanion. The central oxygen atom in [Mo6O19]2- anion and the Si heteroatom in [SiW12O40]4- anion occupy the Oh and Td symmetrical sites respectively, both anions being centrosymmetric with respect to the concerned atoms. Out of the five structures, described in this article, four (1, 3-5) are constituted with the hexamolybdate anion which is a very popular POM anion due to the simplicity of its structure. The anion is known to crystallize with a variety of countercations, for example, alkylammoniums,12 phenylphosphonium and/or arsonium,13 dithiolenes,14 planar aromatic cations,15 transition metal complexes,16 even with crown ether complexes9-11 rendering simple to complicated dimensionalities in molecular packing in the resulting crystalline solids. However, in the present paper, the anion has been introduced not only as a counteranion but also as a supramolecular building block in comprehending the supramolecular interactions between ammonium ion, crown ether, and a POM anion. [H3Oþ⊂(crown ether)] supramolecular complexes further associated with the POM anions have been discussed in the literature,10d,f,11d but, to the best of our

knowledge,17 there is no report, so far, in which an inclusion complex between a crown ether and an NH4þ ion has been engineered with a POM anion. We have described here how the polyoxoanions accumulate the ammonium-crown ether inclusion complexes on their surfaces through supramolecular interactions and build the relevant crystals. Experimental Section Materials and Methods. All the crown ethers (benzo-18-crown-6, dibenzo-18-crown-6, dicyclohexyl-18-crown-6, dibenzo-30-crown10) have been purchased from Sigma-Aldrich and used as received. Acetonitrile solvent, sodium molybdate, and ammonium heptamolybdate have been received from Merck. Tungstosilisic acid was supplied by Alfa-Acer. [Bu4N]2[Mo6O19] has been prepared using a literature procedure.18 FLASH EA series 1112 CHNS analyzer performed elemental analyses. Infrared spectra were recorded as KBr pellets on a JASCO-5300 FT-IR spectrophotometer at 298K. A Cary 100 Bio UV-visible spectrophotometer was used to record the electronic absorption spectra. 1H NMR spectra were recorded in Bruker AV 400 MHz spectrometer. The chemical shifts (δ) are reported in ppm. Powder X-ray diffraction patterns were recorded on a Bruker D8-Advance diffractometer using graphite monochromated CuKR1 (1.5406 A˚) and KR2 (1.54439 A˚) radiations. All the compounds have been prepared via a one-pot synthetic methodology in open reaction vessels. Two different synthetic procedures have been used to prepare compounds 1-5. In cases of 1, 2, and 5, NH4SCN was used as the source of the ammonium ion. The [Mo6O19]2- POM anion for 3 and 4 was generated in situ from the acidified solution of ammonium heptamolybdate. Synthesis of [NH4(B18C6)][Bu4N][Mo6O19] (1). To a stirred solution of benzo-18-crown-6 (0.04 g, 0.13 mmol) in 50 mL of acetonitrile was added [Bu4N]2[Mo6O19] (0.13 g, 0.09 mmol). After all the materials were dissolved, 0.06 g of ammonium thiocyanate (0.79 mmol) was added, followed by the addition of 10 mL of glacial acetic acid (100%). The resulting orange-colored solution was stirred at room temperature for 17 h, filtered, and allowed to evaporate slowly. Orange-colored crystalline blocks appeared after 10 days. They were isolated from the mother liquor by filtration. Yield: 0.025 g (26% based on crown-ether used). Anal. Calcd. for C32H64Mo6N2O25: C, 26.46; H, 4.44; N, 1.93. Found: C, 26.30; H, 4.48; N, 2.01. IR (KBr, cm-1): 3175, 2964, 2872, 1915, 1682, 1591, 1506, 1458, 1417, 1249, 1201, 1120, 952, 798, 592. Synthesis of [NH4(B18C6)]4[SiW12O40] 3 2CH3CN (2). 0.08 g of H4SiW12O40 3 4H2O (0.027 mmol) was dissolved in 50 mL of acetonitrile to which 10 mL of glacial acetic acid was added. To this mixture, was added 0.03 g of benzo-18-crown-6 (0.096 mmol), followed by the addition of 0.1 g of ammonium thiocyanate (1.31 mmol). The resulting pink-colored solution was stirred at room

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Table 1. Crystal Data and Structure Refinement Parameters for 1-3 empirical formula fw T (K), λ (A˚) crystal system space group a (A˚) b (A˚) c (A˚) R () β () γ () V (A˚3) Z, dcalcd (mg m-3) μ (mm-1), F(000) goodness-of-fit on F2 R1/wR2 [I >2σ(I )] R1/wR2 (all data) largest diff peak/hole (e A˚-3)

1

2

3

C32H64Mo6N2O25 1452.49 298 (2), 0.71073 triclinic P1 11.453(3) 11.792(3) 19.729(5) 73.125(4) 73.409(4) 85.848(4) 2443.5(10) 2, 1.974 1.576, 1444 1.017 0.0330/0.0721 0.0480/0.0778 0.590/-0.651

C68H118N6O66SiW12 4309.80 298 (2), 0.71073 triclinic P1 13.4862(18) 15.132(2) 15.324(2) 66.302(2) 70.832(2) 74.559(2) 2671.9(6) 1, 2.688 12.975, 1998 1.399 0.0597/0.1374 0.0635/0.1388 1.288/-2.247

C40H52Mo6N2O35 1692.44 298 (2), 0.71073 monoclinic C2/c 22.4502(14) 11.7332(7) 22.0464(14) 90.00 90.8450(10) 90.00 5806.7(6) 4, 1.936 1.355, 3336 1.097 0.0599/0.1211 0.0817/0.1312 1.141/-0.488

temperature for 15 h, filtered, and allowed to evaporate slowly. Deep yellow block shaped crystals were isolated after 10 days. Yield: 0.04 g (39% based on crown-ether used). Anal. Calcd. for C68H118N6O66SiW12: C, 18.95; H, 2.76; N, 1.95. Found: C, 19.2; H, 2.61; N, 2.1. IR (KBr, cm-1): 3209, 3047, 2903, 2251, 1722, 1593, 1504, 1454, 1321, 1249, 1209, 1122, 970, 920, 792, 530. Synthesis of [NH4(DB18C6)]2[Mo6O19] 3 4H2O (3). 0.035 g of dibenzo-18-crown-6 (0.097 mmol) was dissolved in a mixture of 50 mL of acetonitrile and 10 mL of glacial acetic acid (100%). To this solution, 5 mL of aqueous solution containing 0.05 g of ammonium heptamolybdate (0.04 mmol) was added. The resulting pale yellow colored reaction mixture was then stirred on a magnetic stirrer for 10 min, heated slowly in a mantle to concentrate the volume to almost 40 mL. The deep yellow colored reaction mixture was filtered and kept for crystallization without further disturbances. Orange crystalline blocks appeared after 8 days, which were isolated by filtration. Yield: 0.06 g (73% based on crown-ether used). Anal. Calcd. for C40H52Mo6N2O35: C, 28.38; H, 2.85; N, 1.65. Found: C, 28.44; H, 2.81; N, 1.73. IR (KBr, cm-1): 3526, 3254, 3057, 2901, 1624, 1595, 1504, 1454, 1358, 1325, 1248, 1209s, 1124, 1060, 952, 798, 744, 599. Synthesis of [NH4(DC18C6)]2[Mo6O19] (4). This compound was synthesized using the same procedure as 3. 0.03 g of dicyclohexyl-18crown-6 (0.08 mmol) dissolved in 50 mL of acetonitrile was mixed with 10 mL of acetic acid (100%). To this solution, an aqueous solution (5 mL) containing 0.05 g of ammonium heptamolybdate (0.06 mmol) was added. It was subsequently stirred for 10 min at room temperature and then concentrated to ≈40 mL by slow heating on a heating mantle. It was then filtered and kept for crystallization at room temperature. Yellow plate shaped crystals appeared after two days which was then filtered and dried under a vacuum. Yield: 0.025 g (38% based on crown-ether used). Anal. Calcd. for C40H80Mo6N2O31: C, 28.93; H, 4.85; N, 1.68. Found: C, 29.86; H, 4.83; N, 1.86. IR (KBr, cm-1): 3182, 2937, 1456, 1423, 1356, 1288, 1244, 1205, 1084, 991, 956. Synthesis of [NH4(DB30C10)]2[Mo6O19] 3 CH3COOH (5). 0.03 g of dibenzo-30-crown-10 (0.056 mmol) was dissolved in 50 mL of acetonitrile. To this, were added ammonium thiocyanate (0.14 g, 1.84 mmol) and 0.05 g [Bu4N]2[Mo6O19] (0.04 mmol) and finally 20 mL of acetic acid (100%). The reaction mixture was stirred at room temperature for 18 h and filtered to remove the precipitated white material. The orange colored filtrate, thus obtained, was kept for crystallization in an open condition. Orange-colored crystals were collected by filtration after 15 days. Yield: 0.015 g (25% based on crown-ether used). Anal. Calcd. for. C60H96Mo6N2O43: C, 34.16; H, 4.59, N, 1.33. Found: C, 33.91; H, 4.53; N, 1.38. IR (KBr, cm-1): 3237.77, 3067.09, 2908.92, 1915.49, 1712.94, 1593.35, 1502.68, 1456.39, 1350.29, 1255.77, 1209.48, 1118.81, 1047.44, 956.78, 798.60, 744.59, 601.84, 422.45. X-ray Data Collection and Structure Determination. Single crystals suitable for structural determination of all the compounds (1-5) were mounted on a three circle Bruker SMART APEX CCD area detector system under Mo-KR (λ = 0.71073 A˚) graphite monochromated X-ray beam, crystal to detector distance 60 mm, and a collimator of 0.5 mm. The scans were recorded with an ω scan width of 0.3. Data reduction performed by SAINTPLUS,19a

Table 2. Crystal Data and Structure Refinement Parameters for 4-5 empirical formula fw T (K), λ (A˚) crystal system space group a (A˚) b (A˚) c (A˚) R () β () γ () V (A˚3) Z, dcalcd (mg m-3) μ (mm-1), F(000) goodness-of-fit on F2 R1/wR2 [I >2σ(I )] R1/wR2 (all data) largest diff peak/hole (e A˚-3)

4

5

C40H80Mo6N2O31 1660.70 100 (2), 0.71073 monoclinic P21/n 12.3773(6) 12.1057(6) 20.1711(9) 90.00 104.5980 (10) 90.00 2924.8(2) 2, 1.886 1.337, 1668 1.110 0.0218/0.0513 0.0234/0.0521 0.395/-0.329

C60H96Mo6N2O43 2109.03 100 (2), 0.71073 monoclinic C2/m 16.4726(11) 23.9302(16) 12.1349(8) 90.00 123.6100(10) 90.0 3983.8(5) 2, 1.757 1.013, 2128 1.167 0.0378/0.0944 0.0403/0.0956 0.646/-0.915

empirical absorption corrections using equivalent reflections performed program SADABS,19b structure solution using SHELXS9719c and full-matrix least-squares refinement using SHELXL-9719d for above compounds. All the non-hydrogen atoms were refined anisotropically. Hydrogen atoms on the C atoms of the crown ethers were introduced on calculated positions and were included in the refinement riding on their respective parent atoms. Attempts to locate the hydrogen atoms for the ammonium cations in the crystal structure of compounds 2 and 3 and the same for the solvent water molecules in the crystal structure of compound 3 failed. However, no attempts were made to fix these atoms on their parents. Crystal data and structure refinement parameters for all the compounds (1-5) are presented in Tables 1 and 2.

Results and Discussion Synthesis. Single crystals for the title compounds 1-5 have been grown from acidified organic (compounds 1, 2, and 5) or acidified aqueous-organic medium (compounds 3-4) using two different synthetic routes. For compounds 1, 2, and 5, the readymade POM cluster precursors [Bu4N]2[Mo6O19] (compounds 1 and 5) and H4SiW12O40 (compound 2) have been reacted with crown ether and NH4SCN (as the NH4þ ion source). The POM anions are reasonably stable in the reaction condition and crystallize after their association with the [NH4þ⊂(crown ether)] cations (a supramolecular cationic species). On the other hand, for compounds 3 and 4, ammonium heptamolybdate has been utilized to serve for both the NH4þ ion source as well as to generate the isopolyanion [Mo6O19]2- in situ from acidified MeCNH2O medium. Although various POM anions can be assembled

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Figure 1. Normalized UV-visible spectra of the compounds 1-3 in DMSO recorded at 298 K.

in this reaction condition, the probable reason for the selectivity of the macrocyclic cation toward hexamolybdate anion is not so clear to us. It is notable that we could not isolate single crystals for compounds 3 and 4 using the reaction condition described for compounds 1, 2, and 5 and vice versa. Spectroscopy. UV-Visible Spectroscopy. Compounds 1 and 2 consist of the same crown ether, that is, B18C6, but different POM anions, that is, [Mo6O19]2- and [SiW12O40]4- for compounds 1 and 2 respectively, whereas compound 3 contains DB18C6 macrocycle and [Mo6O19]2- POM anion. To check the outcome of varying crown ether or POM anion on the absorption properties of the relevant compounds, we have recorded UV-visible spectra of compounds 1-3 in DMSO (concentration ∼ 10-4 M) at 298 K. The corresponding spectra have been presented in Figure 1. Crystals of the compounds 1 and 3 are orange-red in color while that of the compound 2 are yellow. But in the relevant spectra no band in the visible region of the spectra is observed. Instead, a structureless broad band is observed in the ultraviolet region of the spectra (compounds 1 and 3) centered at ca. 320 nm. For the compound 2, a broad shoulder originating at 360 nm is observed (Figure 1). Thus, changing the crown ether from B18C6 to DB18C6 has not altered the absorption property of the corresponding compounds in solution state, whereas a significant change has been noticed when the POM anion is changed from [Mo6O19]2- (in 1) to [SiW12O40]4- (in 2). Infrared Spectroscopy. Infrared spectroscopy is a useful tool for the detection of particular functional groups. Free ammonium ion (NH4þ) has Td symmetry, which usually displays a strong and broad absorption in the region of 3300-3030 cm-1 because of N-H stretching vibrations. Another strong band near 1400 cm-1 is normally observed due to the N-H bending motions.20 These bands shift to higher and lower wavelengths respectively when the relevant ion is involved in H-bonding. Thus, in compounds 1-5, inclusion phenomenon of NH4þ ion in the crown ether cavity is easily realized by means of IR spectroscopy, as the ion is assimilated within the crown ether cavity through hydrogen bonding interactions. Appearance of a broad vibrational band centered at 3530 cm-1 in the IR spectrum of the compound 3 suggests the presence of solvent/ H-bonded water molecule in the compound. Strong and broad bands at 3175, 3209, 3254, 3182, 3238 cm-1 (Figure 2) for the compounds 1-5 respectively are assigned to the N-H stretching absorption of the NH4þ ion; the variation in band positions is possibly due to differences in H-bonding environment around

Figure 2. IR spectra of the compounds 1-5 in the high frequency region (solid state, KBr pellets). Numbers in the parentheses at the left side of each spectrum correspond to the relevant compound.

the ion. The IR bands at 1458, 1454, 1454, 1456, 1456 cm-1 for compounds 1-5 respectively are attributed to the N-Hdef vibrations of the NH4þ ion. Multiple bands in the region of 30752950 cm-1 are due to various C(Ar)-H, C(sp3)-H along with combined N-H stretching vibrations in the compounds 1-5. A weak intensity band at 2251 cm-1 in the spectrum of compound 2 resembles -CN stretching of aliphatic nitrile. A sharp IR band of medium intensity at 1712 cm-1, in the spectrum of compound 5, is attributed to the >CdO stretching of saturated aliphatic acid dimer (that is observed indeed in the relevant crystal structure), but the associated O-H stretching (which generally appears at ≈3400-2800 cm-1 as a broad band) is not distinctly observed in the spectrum, probably due to its merging with other vibrations. For the compounds 1, 2, 3 and 5, the aromatic CdC bond stretching vibrations of the associated phenyl ring of crown ethers have been observed as two strong sharp bands in the regions of 1595-1590 and 1506-1504 cm-1. These bands are not observed in the spectrum of compound 4, as it does not contain any aromatic moiety. The out-of-plane C-H bending vibrational band for the o-substituted aromatic ring at 744 cm-1 is detected in the IR spectra of compounds 3 and 5 (crown ethers DB18C6 and DB30C10 respectively), whereas a mixture of this vibrational band with υ(M-O-M) (M=Mo, W)

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Figure 3. 1H NMR spectra of the compounds 1-4 showing shift and/or splitting of the NH4þ signals with respect to NH4Cl.

vibration results in strong and broad absorptions in the range of 800-790 cm-1 for compounds 1, 2, and 4. Strong sharp bands in the region 950-970 cm-1 in the IR spectra of the compounds 1-5 are attributed to MdO stretching vibration of the [Mo6O19]2- or [SiW12O40]4- anions. IR spectra of the compounds 1-5 in the high frequency region are presented in Figure 2. 1 H NMR Spectroscopy. 1H NMR spectra of compounds 1-4 have been recorded in DMSO-d6 at room temperature. For comparison purposes, we have recorded the same for NH4Cl. Selected region for the spectra of the relevant compounds are presented in Figure 3 (see Supporting Information for full spectra). NH4Cl shows a sharp singlet at δ 7.49 in DMSO, whereas there is profound variation in the nature of NH4þ ion resonances in the spectra of compounds 1-4. Intramolecular interactions between the NH4þ ion and the crown ethers inside the relevant hydrogen bonded complexes are visually observed in the solid state through X-ray diffraction technique. There is a certain possibility of retention of these interactions in the solution state too, because of their invariance (unlike intermolecular interactions) with the molecular concentration in solution. Thus, the relevant matter should be realized through 1H NMR spectral studies with respect to chemical shift of the NH4þ ion in an aprotic solvent. Although it is known that high polar solvents such as DMF or DMSO have the chance to break ionic assemblies, we were constrained to record the spectra of the relevant compounds in DMSO only because of the poorer solubility of these compounds in MeCN, etc. Hence, a firm conclusion about the effect of the POM anion on the chemical shift/signal splitting of the NH4þ ion in the relevant spectra should not be drawn. However, a clear distinction of the corresponding resonances with respect to NH4Cl proves the retention of the intramolecular interactions inside the [NH4þ⊂(crown ether)] supramolecular complexes in solution, the variation of chemical shift being due to difference in the electronic environment around the ion. In all the cases, resonance due to the concerned ion has occurred at higher fields with respect to NH4Cl. The probable reason for this up-field shifting of the signals might be due to a shielded environment around the ion inside the crown ether cavity (due to lone pair on O atoms etc.).


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As shown in Figure 3 (in case of compounds 1 and 3), the resonance has occurred twice with (approximately) 1:1 relative intensity (δ 7.12, 7.10 for compound 1 and δ 7.12, 7.05 for compound 3, respectively) of the signals, whereas for compound 4 it has appeared thrice (δ 7.03, 6.90, 6.77) with ca. 1:1:1 relative intensity ratio. For compound 2, a broad signal at δ 7.01 has been observed. Tetrahedrally symmetrical NH4þ ion should resonate once, while the multiple resonances of the encapsulated NH4þ ion correspond to the breakage of its symmetry due to the presence of H-bonding interactions with the crown ethers in solution. Ammonium Ion or Hydroxonium Ion? 18-crown-6 and its derivatives are known to be good stabilizing ligands for hydroxonium ion (H3Oþ).3 Hydrogen atoms for the ammonium cations in compounds 2 and 3 were not located from the difference Fourier map (see Crystal Structure Description section, vide infra). Thus, an ambiguity might appear about the cation: whether it is ammonium ion (NH4þ) or hydroxonium ion (H3Oþ)? In this context, it should be mentioned that, a clearer distinction between a free O atom and a N atom cannot be firmly established with the help of crystallography (in presence of heavy scatterers e.g. heavy metal atoms) unless the relevant H atoms are located in their crystal structures. Both the atoms (N and O) have almost identical X-ray scattering abilities. As such, a wrong assignment of the electron density in the difference Fourier map does not alter the structural refinement minimization parameters (R1, wR2, shift/esd, etc.) to a considerable extent. Wang and co-workers reported the isolation of [H3Oþ⊂18C6]10d and [H3Oþ⊂ DB18C6]10f supramolecular crown ether adducts in the PMo12O403- POM matrix from a mixture of crown ether and H3PMo12O40 in MeCN or MeOH-MeCN mixture. Hydroxonium ion in its crown ether complexes generally shows a broad IR absorption at ≈2950 cm-1 (overlapped with C-H stretching) characteristic for the O-H stretching vibration.10d,f,21a Another typical band appears at ≈1098 cm-1 that corresponds to the C-O-C stretching of the crown ether coupled with the O-H bending motion of the hydroxonium ion.21b The characteristic broad IR absorption for hydroxonium ion (that generally appears around 2950 cm-1), as mentioned above, is clearly missing in our compounds 2 and 3 (Figure 2). This indicates that NH4þ ion (not H3Oþ) is encapsulated in the crown ether cavity of compounds 2 and 3 as the cation. Moreover, the characteristic IR absorption band for the NH4þ ion appears at 3300-3030 cm-1 (N-H stretching) and ≈1400 cm-1 (N-H bending). Thus, the bands at 3209 and 3254 cm-1 in the IR spectra of compounds 2 and 3 respectively strongly suggest the presence of ammonium ions (N-H stretching) in the crystalline material. The matter is further supported by the appearance of IR bands at 1454 cm-1 (N-H bending) for both the compounds. Thus, any possibility to include H3Oþ cation instead of the NH4þ cation in the crown ether cavity is ruled out. Description of Crystal Structure. Compound [NH4(B18C6)][Bu4N][Mo6O19] (1). Compound [NH4(B18C6)][Bu4N][Mo6O19] (1) crystallizes in P1 space symmetry (triclinic, Z0 =1) from the MeCN-CH3COOH solution. The structure consists of two different cations viz. [NH4(B18C6)]þ and [Bu4N]þ along with the [Mo6O19]2- isopolyanion in its asymmetric unit. Attempts for complete replacement of the tetrabutylammonium cation using excess of ammonium thiocyanate has remained unsuccessful. Thus, there is an unwanted perturbation from the tetrabutylammonium cation with the molecular and/or crystal symmetry of compound 1. The anticipated structure would contain two ammonium-crown ether supramolecular cations along with the

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Figure 4. (a) Thermal ellipsoidal plot of the compound 1 in 20% probability distribution; (b) hydrogen bonding environment in the crystal structure. Symmetry operations to relate the atoms with additional labels (#) are in accord with the footnote of Table 3; (c) supramolecular chainlike assembly of compound 1 viewed parallel to the crystallographic [101] plane. C-H 3 3 3 π interactions between the crown ethers are visualized in the picture. H-atoms are omitted for clarity. Color code: C, light gray; H, pink; N, blue; O, red; [Mo6O19]2-, red polyhedra. The same color convention is followed for rest of the color figures.

[Mo6O19]2- anion for electro neutrality. Accordingly, the expected structure of the molecule might be a centrosymmetric one with the center of inversion being the central oxygen atom of the hexamolybdate anion. However, this did not happen. The crystal structure of compound 1 is shown in Figure 4a. The [Mo6O19]2isopolyanion (also known as Lindqvist type polyoxometalate anion) is a symmetrical, almost spherical polyoxometalate, with all the Mo atoms at þVI oxidation states. Each Mo atom attains a distorted octahedral geometry coordinating to one central (Oc), one terminal (Ot), and four bridging (Ob) oxygen atoms and the polyoxoanion is formed by six edge shared {MoO6} octahedra. The Mo-O bond lengths can be grouped into three sets: Mo-Ot 1.674-1.680 A˚, Mo-Ob 1.876-1.932 A˚, and Mo-Oc 2.2992.328 A˚. The bond angles of Mo-Ob-Mo are observed in the range of 116.0-117.18, while that of Mo-Oc-Mo are 89.4490.60 and 179.5-179.78 in the relevant crystal structure. On the other hand, B18C6 is a macrocyclic polyether consisting of six O atoms that falls into two categories: (a) catechol oxygen atoms, that is, O atoms attached to the phenyl ring and (b) glycolic oxygen atoms, that is, atoms attached to the ethylene chains. In the crystal structure of compound 1, the various bond distances for the crown ether B18C6 has been observed as follows: C(aryl)-O(aryl) 1.360-1.373 A˚, C(alkyl)-O(aryl) 1.412-1.434 A˚, C(alkyl)-O(alkyl) 1.321-1.420 A˚, C(aryl)C(aryl) 1.352-1.389 A˚, and C(alkyl)-C(alkyl) 1.243-1.481 A˚

(see Table 5). 1:1 inclusion complexation between the ammonium ion and the crown ether B18C6 has been observed in the crystal structure of compound 1 and the macrocycle is found to be slightly bent outward from the cation (see Figure 4). The NH4þ guest cation has been included in the crown ether cavity through six bifurcating Nþ-H 3 3 3 O hydrogen bonding interactions with the crown ether oxygen atoms O(1) to O(6) using three N-H bonds. The fourth N-H proton is projected outward from the crown ether cavity and is involved in hydrogen bonding interaction with the O(23), bridging oxygen atom of the associated [Mo6O19]2- isopolyanion. This type of interaction between the ammonium-crown ether supramolecular complex and the POM anion has already been proposed in Scheme 1. C-H 3 3 3 O interaction between the crown ether and the POM cluster anion [C(8)-H(8A) 3 3 3 O(19)] has also been demonstrated in the crystal structure (Figure 4b). In a nutshell, the POM cluster anion holds both the crown ether (C-H 3 3 3 O interaction) and the NH4þ ion (Nþ-H 3 3 3 O interaction) in suitable positions through supramolecular interactions. The tetrabutylammonium cation is involved in C-H 3 3 3 O interactions with three POM anions using C(20), C(21), C(26), and C(29) atoms of the cation (see Figure 4b). H-bonding parameters for 1 have been summarized in Table 3. Figure 4c clearly illustrates how the nature of these supramolecular interactions paves the way toward supramolecular assembly of a chainlike arrangement. Two inversion symmetry

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Crystal Growth & Design, Vol. 10, No. 7, 2010 Table 3. H-Bonding Parameters for the Compound 1a

D-H 3 3 3 A C(8)-H(8A) 3 3 3 O(19) C(20)-H(20A) 3 3 3 O(24)#2 C(21)-H(21A) 3 3 3 O(14)#1 C(26)-H(26A) 3 3 3 O(16) C(29)-H(29A) 3 3 3 O(22) N(1)-H(2N) 3 3 3 O(23) N(1)-H(4N) 3 3 3 O(1) N(1)-H(4N) 3 3 3 O(2) N(1)-H(3N) 3 3 3 O(3) N(1)-H(3N) 3 3 3 O(4) N(1)-H(1N) 3 3 3 O(5) N(1)-H(1N) 3 3 3 O(6)

d(D-H) d(H 3 3 3 A) d(D 3 3 3 A) 0.97 2.46 3.328(5) 0.96 2.53 3.452(6) 0.97 2.46 3.361(5) 0.97 2.59 3.448(5) 0.97 2.40 3.215(5) 0.85(6) 2.24(7) 3.085(6) 0.73(5) 2.32(5) 2.923(6) 0.73(5) 2.28(5) 2.932(6) 0.94(8) 2.16(8) 2.913(6) 0.94(8) 2.16(8) 2.947(6) 0.97(7) 2.12(7) 2.997(7) 0.97(7) 2.18(7) 2.905(6)

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Table 4. H-Bonding Parameters for the Compound 2a

— (DHA) 148.4 160.5 155.2 147.1 141.2 172(6) 141(5) 150(5) 136(6) 140(7) 150(6) 131(5)

D-H 3 3 3 A d(D-H) d(H 3 3 3 A) d(D 3 3 3 A) — (DHA) C(7)-H(7A) 3 3 3 O(5)#2 0.97 2.64 3.54(3) 153.4 C(12)-H(12B) 3 3 3 O(8)#4 0.97 2.67 3.60(3) 162.1 C(14)-H(14B) 3 3 3 O(23)#3 0.97 2.55 3.37(3) 141.5 C(16)-H(16B) 3 3 3 O(31)#1 0.97 2.67 3.54(3) 149.5 C(23)-H(23B) 3 3 3 O(25)#6 0.97 2.54 3.44(2) 153.8 C(24)-H(24A) 3 3 3 O(23)#1 0.97 2.53 3.39(2) 146.9 C(24)-H(24B) 3 3 3 O(32)#5 0.97 2.67 3.54(2) 149.6 C(29)-H(29A) 3 3 3 O(18) 0.97 2.56 3.38(3) 142.9 C(30)-H(30B) 3 3 3 O(17)#1 0.97 2.67 3.59(4) 158.8 C(32)-H(32A) 3 3 3 O(28)#3 0.97 2.69 3.55(3) 148.0 C(34)-H(34B) 3 3 3 O(12)#1 0.96 2.62 3.54(4) 161.2

Symmetry transformations used to generate equivalent atoms: #1 x - 1, y, z; #2 -x þ 1,-y þ 1,-z þ 1.

a Symmetry transformations used to generate equivalent atoms: #1 -x þ 1, -y þ 1, -z þ 1; #2 -x þ 2, -y þ 1, -z þ 2; #3 x, y þ 1, z; #4 x þ 1, y, z; #5 -x,-y þ 1,-z þ 1; #6 x - 1, y þ 1, z.

related molecules of compound 1 forms a hydrogen bonded dimer through C-H 3 3 3 O interactions between the tetrabutylammonium cation and the POM cluster anion thus builds the basic supramolecular hydrogen bonded unit (ammonium-crown ether)þ-POM-2[Bu4N]þ-POM-(ammonium-crown ether)þ (see Figure 11 in the Supporting Information). These dimers are, in turn, connected through C(21)-H(21A) 3 3 3 O(14) hydrogen bond and its symmetry complement (inversion) along the crystallographic a-axis resulting in the formation of a supramolecular chainlike topology in the relevant crystal structure (see Figure 4c). Packing of the molecules in the crystal lattice of 1 can be further characterized by the abundance of C-H 3 3 3 π and π 3 3 3 π interactions among the crown ethers. The combination of C-H 3 3 3 π interaction between C(12)-H(12B) of one crown ether and the π-ring of its translation equivalent with π 3 3 3 π interactions between the phenyl rings of two inversion symmetry related crown ethers results in stacking of the macrocyles parallel to the crystallographic a-axis as shown in Figure 5. Tetrabutylammonium cations and the POM anions fill up the space between two such translation related crown ether only chains resulting in the complicated three-dimensional crystal packing of compound 1 (see Figure 12 in the Supporting Information). Compound [NH4(B18C6)]4[SiW12O40] 3 2CH3CN (2). The crystal structure of compound [NH4(B18C6)]4[SiW12O40] 3 2CH3CN (2) consists of [SiW12O40]4- as the counter-anion, which is known as R-Keggin type heteropolyanion. The compound 2 crystallizes as an acetonitrile solvate, and its crystal structure is characterized by four ammonium-crown ether supramolecular complex cations associated with one [SiW12O40]4- anion (Z0 =1/2) (see Figure 6a). The Si(1) atom of the POM cluster anion is located at the center of crystallographic ac plane (xyz = 0.5, 0, 0.5) with half occupancy and is the inversion center of the molecule. The crystallographically observed [SiW12O40]4- POM anion in the crystal structure of compound 2 is best described as a symmetrically distorted RKeggin type structure (see Figure 13 in the Supporting Information).10a,22 All the W atoms of [SiW12O40]4- anion reside in distorted octahedral geometry {WO6} coordinating to two terminal (Ot), four bridging (Ob) and two central (Oc) oxygen atoms (see Table 5 for experimentally observed various bond lengths). In the crystal structure of the compound 2, the POM anion accepts four ammonium-crown ether supramolecular cations at its surface both by Coulombic as well as supramolecular H-bonding interactions (see Table 4 for the related H-bonding parameters and Figure 14 in the Supporting Information for pictorial representation). Although the directional nature of H-bonding interactions between the ammonium ion and the POM remains undetermined (H atoms of the cation are

not located), short N 3 3 3 O contact N(2) 3 3 3 O(20) [3.033(10)A˚, symmetry 1 - x, 1 - y, 1 - z)] suggests the supramolecular interaction between them. Inside the ammonium-crown ether adducts, the N-O distances are found to be in the range of 2.798(8)-3.01(2) A˚ (see Table 5). In contrast to compound 1, compound 2 consists of two different types of crown ether adducts as far as puckering of the macrocycle is concerned. In one ammonium-crown supramolecular cation (consisting of O(7) to O(12)), the crown ether is twisted outward from the ammonium ion and in the other one (consisting of O(1) to O(6)), the macrocycle is curved inward to the ammonium cation as shown in Figure 6b. In the crystal structure, the basic supramolecular units, 4(NH4-crown)þ-SiW12O404- are further connected by C(29)-H(29A) 3 3 3 O(18) interaction and their symmetry equivalent partners, resulting in the formation of a supramolecular chain that propagates parallel to the crystallographic b-axis (Figure 6b). Each of such chains are again connected by means of C(23)-H(23B) 3 3 3 O(25) supramolecular weak interactions along with their symmetry equivalents parallel to the crystallographic c-axis. However, no C-H 3 3 3 π or π 3 3 3 π interactions are observed among the crown ethers in the crystal lattice of compound 2, possibly because the larger surface area of the POM anion impedes the crown ethers from getting involved in such type of stacking interaction due to the dearth of space. Compound [NH4(DB18C6)]2[Mo6O19] 3 4H2O (3). Replacing the less symmetrical crown ether B18C6 with a more symmetrical one DB18C6 has resulted in a different mode of crystal packing in compound [NH4(DB18C6)]2[Mo6O19] 3 4H2O (3) compared to the last two crystal structures (compounds 1 and 2). The compound 3 crystallizes as a hydrate in a rather higher symmetrical space group (C2/c) than compounds 1 and 2 (both crystallized in P1 space symmetry). The asymmetric unit in the crystal structure of compound 3 is characterized by two symmetry independent half ammonium-crown ether supramolecular complex cations, half of the [Mo6O19]2- anion and two solvent water molecules (Z0 = 1/2). Both the N(1) and N(2) atoms of the ammonium cations are found at the concave sides of the crown ethers (Figure 7a), and they reside along the two-fold symmetry axis of a monoclinic unit cell with half occupancies. Similarly, the central oxygen atom of the POM anion O(2) is positioned at the inversion center (xyz=0.25, 0.25, 0.50) of the crystal. Figure 7a depicts the crystal structure of compound 3. In the relevant crystal structure, the bond lengths for the crown ether (DB18C6) are observed as follows: C(aryl)-C(aryl) 1.347-1.392 A˚, C(alkyl)-C(alkyl) 1.467-1.495 A˚., C(aryl)-O(aryl) 1.3781.391 A˚, C(aryl)-O(alkyl) 1.427-1.429 A˚ and C(alkyl)O(alkyl) 1.413-1.439 A˚ (see Table 5). As the H-atoms of

a

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Table 5. Selected Bond Lengths for the Crystal Structure of the Compounds 1-3 Compound 1 C(1)-C(2) C(4)-C(5) C(7)-C(8) C(13)-C(14) C(6)-O(2) C(9)-O(3) C(12)-O(5) C(15)-O(6)

1.380(6) 1.379(7) 1.481(6) 1.409(9) 1.374(5) 1.410(5) 1.414(7) 1.332(7)

C(2)-C(3) C(5)-C(6) C(9)-C(10) C(15)-C(16) C(7)-O(2) C(10)-O(4) C(13)-O(5) C(16)-O(1)

1.381(7) 1.371(6) 1.476(7) 1.243(8) 1.434(5) 1.407(6) 1.321(7) 1.412(6)

C(3)-C(4) C(1)-C(6) C(11)-C(12) C(1)-O(1) C(8)-O(3) C(11)-O(4) C(14)-O(6)

1.352(7) 1.389(6) 1.477(7) 1.361(5) 1.419(5) 1.411(6) 1.321(8)

Compound 2

C(1)-C(2) C(4)-C(5) C(7)-C(8) C(13)-C(14) C(6)-O(2) C(9)-O(3) C(12)-O(5) C(15)-O(6)

1.35(3) 1.37(3) 1.45(3) 1.46(3) 1.36(2) 1.47(3) 1.45(3) 1.39(2)

Crown Ether 1 C(2)-C(3) C(5)-C(6) C(9)-C(10) C(15)-C(16) C(7)-O(2) C(10)-O(4) C(13)-O(5) C(16)-O(1)

1.42(3) 1.36(3) 1.41(4) 1.48(3) 1.48(2) 1.45(3) 1.42(3) 1.44(2)

C(3)-C(4) C(1)-C(6) C(11)-C(12) C(1)-O(1) C(8)-O(3) C(11)-O(4) C(14)-O(6)

1.32(3) 1.40(3) 1.45(3) 1.39(2) 1.42(3) 1.42(3) 1.42(2)

C(17)-C(18) C(20)-C(21) C(17)-O(12) C(23)-C(24) C(25)-O(8) C(27)-O(9) C(29)-O(10) C(31)-C(32)

1.34(3) 1.39(3) 1.40(2) 1.47(3) 1.38(3) 1.32(3) 1.45(4) 1.46(3)

Crown Ether 2 C(18)-C(19) C(21)-C(22) C(22)-O(7) C(24)-O(8) C(26)-O(9) C(28)-O(10) C(30)-O(11) C(32)-O(12)

1.37(4) 1.35(3) 1.38(2) 1.42(2) 1.28(4) 1.41(4) 1.40(3) 1.41(2)

C(19)-C(20) C(17)-C(22) C(23)-O(7) C(25)-C(26) C(27)-C(28) C(29)-C(30) C(31)-O(11)

1.39(4) 1.39(3) 1.45(2) 1.36(4) 1.28(4) 1.39(4) 1.41(3)

N1 3 N1 3 N1 3 N2 3 N2 3

3.00(2) 2.94(2) 2.798(8) 3.00(3) 2.93(2)

N1 3 N1 3 N2 3 N2 3

3 O3 3 O6 3 O8 3 O11

3.01(2) 2.90(2) 2.82(2) 3.01(2)

3 3 3 3 3

3 O1 3 O4 3 O17 3 O9 3 O12

N-O Distances (NH4þ-Crown Ether) N1 3 3 3 O2 3.10(2) N1 3 3 3 O5 3.00(2) N2 3 3 3 O7 3.009(18) N2 3 3 3 O10 2.85(2)

3 3 3 3

Compound 3

C(2)-C(3) C(5)-C(6) C(1)-O(11) C(8)-O(12) C(10)-O(13)

1.384(9) 1.387(10) 1.439(8) 1.425(8) 1.438(8)

C(11)-C(12) C(14)-C(15) C(15)-O(14) C(18)-O(15)

1.368(15) 1.347(12) 1.391(10) 1.421(11)

Crown Ether 1 C(3)-C(4) C(6)-C(7) C(2)-O(11) C(8)-C(9)

1.369(11) 1.380(10) 1.378(8) 1.483(10)

C(4)-C(5) C(2)-C(7) C(7)-O(12) C(9)-O(13)

1.361(11) 1.383(9) 1.378(8) 1.413(8)

Crown Ether 2 C(12)-C(13) C(16)-C(17) C(16)-O(14) C(19)-O(16)

1.372(16) 1.467(13) 1.436(9) 1.418(9)

C(13)-C(14) C(18)-C(19) C(17)-O(15) C(20)-O(16)

1.390(13) 1.495(13) 1.438(11) 1.368(11)

both the ammonium ions as well as the solvent water molecules are not located in the relevant crystal structure, the direction/nature of the supramolecular interactions between the NH4þ-crown ether complex cations, solvent water molecules and POM anions are not so clear. But a qualitative idea can be drawn from the concerned short N 3 3 3 O and O 3 3 3 O distances (see Table 7). Inside the NH4þ-DB18C6 supramolecular adducts, six short N 3 3 3 O distances are observed in the range of 2.977-3.043 A˚ that represent strong interactions between the ammonium cations and the crown ethers. In the crystal structures of compounds 1 and 2, direct interaction between the ammonium ion and the POM anions have been demonstrated which resembles the expectation as postulated in Scheme 1. But in the crystal structure of compound 3, no direct contact between the ammonium cation and the POM anion has been observed. Possibly the bowl shape of the crown ether DB18C6 has made this difference. The lattice water molecule O(17), which is located in between two orthogonally arranged supramolecular

crown ether cationic complexes, joins them through N(1) 3 3 3 O(17) [2.798(8) A˚], O(17) 3 3 3 O(14) [3.059(8) A˚] and O(17) 3 3 3 O(16) [3.118(9) A˚] supramolecular weak interactions (see Figure 15 in the Supporting Information). Translation of these interactions results in a chainlike topology for the arrangement of the crown ether complexes parallel to the crystallographic b-axis (see Figure 7b). Two such oppositely directed chains (which are related by the inversion symmetry) are bridged by the POM anion through C(12)-H(12) 3 3 3 O(10) [3.607(9)A˚] supramolecular interaction as shown in Figure 7b (see Table 7 for the parameters for these interactions). It has already been described that the crystal structure of compound 3 consists of two differently oriented symmetry independent crown ethers (see Figure 7a). Thus, two dissimilar types of packing of the crown ethers are expected in the relevant crystal lattice. Reflection symmetry related crown ethers parallel to the crystallographic c-axis along with their symmetry equivalents (rotation and translation) pack in a channel-like arrangement (Figure 8).

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Figure 5. C-H 3 3 3 π and π 3 3 3 π stacking interactions between the crown ethers parallel to the crystallographic [100] axis in the crystal structure of the compound 1. Geometrical parameters for these interactions are as follows: for C-H 3 3 3 π interaction, d(H12B 3 3 3 Cg) = 3.134(4), d(C12 3 3 3 Cg) = 4.068 (9)A˚, — (C12-H12B 3 3 3 Cg) = 162.01(17) (symmetry: 1 þ x, y, z); for π 3 3 3 π interaction, d(Cg-Cg) = 3.632(3)A˚, R = 0.02 (symmetry: 2 - x, 1 - y, -z), where Cg refers to the centroid of the concerned phenyl ring.

Figure 6. (a) Crystal structure of compound 2 with atom labeling scheme in 40% thermal probability distribution. Only the asymmetric unit has been displayed. Hydrogen atoms and the solvent MeCN molecule are omitted for a clearer view; (b) formation of supramolecular chain as a result of supramolecular interactions between the crown ethers and the POM anions viewed down the crystallographic [001] axis.

Another chain of C-H 3 3 3 π stacked crown ethers runs across this channel, that is, perpendicular to the crystallographic abplane (see Figure 8). The resultant packing of the molecules in the crystal structure of compound 3 is beautiful and symmetrical as shown in Figure 7c. The packing nature of compound 3 is found to be almost identical with the crystal structure of bis-(tetrachlorodibenzo-18-crown-6) tetrachloro-aurate(III) oxonium tetrahydrate (CCDC depository code. QUQPEX) described by Steed and co-workers.23a Space group of the same was incorrectly assigned to Cc, which was further correctly assigned to C2/c by Marsh.23b Compound [NH4(DC18C6)]2[Mo6O19] (4). Dicyclohexyl-18-crown-6 (DC18C6) is the saturated analogue of

dibenzo-18-crown-6 (DB18C6). On the basis of the fusion of the cyclohexane rings (cis or trans) and the relationship between the two (syn or anti), this crown ether exists as five stereoisomers24,25 as shown in Scheme 2. The crown ether DC18C6 contains two cyclohexyl rings which are nonplanar, nonaromatic in contrast to the planar and aromatic phenyl rings of its aromatic analogue DB18C6. Depending upon the nature of stereoisomerization of the crown ether, the two oxygen atoms on the same cyclohexane ring can either be both axially or equatorially positioned or one might be in an axial position and another in an equatorial position (or vice versa), whereas the catechol oxygen atoms of the macrocycle DB18C6 are in plane with the phenyl rings. As the crown

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Figure 7. (a) Thermal ellipsoidal plot of the compound 3 in 20% confidence level; (b) interconnection between two oppositely directed supramolecular chains through polyoxometalate bridge. These chains are related to each other by the inversion symmetry; (c) packing of the molecules in the crystal lattice of compound 3 viewed down the crystallographic [010] axis.

ether DC18C6 does not contain any π-ring, no π-stacking interactions would be present in the crystal lattice of compound [NH4(DC18C6)]2[Mo6O19] (4) and thus, the packing mode of the molecules would obviously deviate from that of the compounds 1 and 3. The molecules of compound

[NH4(DC18C6)]2[Mo6O19] (4) assemble in P21/n space symmetry with one ammonium-crown ether supramolecular complex cation and half of the [Mo6O19]2- anion in its asymmetric unit (Z0 =1/2). An attempt to refine the structure in the standard symmetry P21/c has resulted in unusually poor data

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convergence (with high shift/esd), so the space group of the relevant crystal has been assigned to P21/n only. The crystal structure of the compound 4 has been displayed in Figure 9a. The POM anion is situated at the corner of the crystallographic ac plane, the central oxygen atom of the POM, that is, O(10) being the inversion center (xyz = 1.0, 0.0, 1.0) of the molecule. The oxygen atoms attached to the cyclohexane rings of the crown ether are found to face upward and the rings also project to the same direction thereby making the crown ether look like an umbrella with cis-syn-cis conformation. In this context, it should be mentioned that the commercially procured DC18C6 (Sigma-Aldrich) was a mixture of the isomers, while in the resulting crystal structure only one stereoisomer of the crown ether is found (see PXRD discussions). The reason for this selectivity is not clear to us. As seen in Figure 9a, the crown ether is highly puckered and the ammonium ion is located at a distance of 1.0231(2) A˚ from the mean macrocycle {O6} plane. O(12) and O(14) atoms of the crown ether are in axial positions, while O(11) and O(15) are in the equatorial positions with respect to the two cyclohexane rings (see Figure 9b), the rings being in chair conformations. In the crystal structure of compound 4, the bond lengths of the crown ether are observed as follows: C(cyclohexyl)-C(cyclohexyl) 1.515-1.534 A˚ which are longer than the C(alkyl)-C(alkyl) bonds 1.495-1.502 A˚; C(cyclohexyl)-O(axial) 1.433-1.437 A˚ which are shorter than

Figure 8. Arrangement of crown ethers in the crystal lattice of compound 3. Two distinct arrangements for the crown ethers are clearly visible. Parameters for C-H 3 3 3 π interactions. d(H1B 3 3 3 Cg) = 3.20(10) A˚, d(C1 3 3 3 Cg)=4.137(7), — (C1-H1 3 3 3 Cg)=162.90(19). Symmetry: 0.5 - x, 0.5 - y, 1 - z.

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the C(cyclohexyl)-O(equatorial) bonds 1.438-1.441 A˚ and the C(alkyl)-O bonds in the range of 1.422-1.432 A˚ (see Table 6). The guest cation (ammonium) is incorporated in the crown ether cavity through three strong Nþ-H 3 3 3 O hydrogen bonding interactions with O(11), O(13), and O(15) of the macrocycle, and interacts with the POM anion through N(1)-H(1N) 3 3 3 O(4) hydrogen bond donation (see Scheme 1). Nþ-H 3 3 3 O interaction between N(1) 3 3 3 O(4) followed by the inversion symmetry with respect to O(10) generates the full molecule where the POM anion is sandwiched between two supramolecular ammonium-crown ether adducts (see Figure 9c). These supramolecular sandwiches are further supported by the CH 3 3 3 O interaction between C(14) of the crown ether and O(5) of the POM anion. H-bonding parameters in the crystal structure of compound 4 have been tabulated in Table 8. Each of these supramolecular sandwiches are again connected to molecules of the same kind to form a zig-zag ribbon-like arrangement via C(20)-H(20B) 3 3 3 O(6) [3.386(3) A˚], C(17)-H(17A) 3 3 3 O(2) [3.352(3) A˚], and C(14)-H(14B) 3 3 3 O(9) [3.459(3) A˚] hydrogen bonding interactions as shown in Figure 9c. Compound [NH4(DB30C10)]2[Mo6O19].CH3COOH (5). Unlike compounds 1-4, compound 5 comprises the larger crown ether dibenzo-30-crown-10 (DB30C10) that contains 16 sp3 hybridized C atoms, and so, it is structurally the most flexible one among all the crown ethers described in this article. This larger macrocycle (DB30C10) can swallow a cation either by wrapping around it or by occupying more than one cation in its cavity.26 Single crystal structure of the crown ether DB30C10 shows a chairlike conformation of the crown ether which can best be described as a quasi-planar form.26g However, after interacting with a cation, the crown ether takes up different conformations depending on the nature of binding. Encapsulation of metal cations or hydroxonium ions,26a-h even substituted ammonium ions26i,j in the cavity of DB30C10 has been discussed in the literature. Here, in the case of compound [NH4(DB30C10)]2[Mo6O19] 3 CH3COOH (5), we have shown the effect of the Nþ-H 3 3 3 O hydrogen bonding interaction for binding a single NH4þ ion in the big cavity of DB30C10. The title compound 5 crystallizes in the monoclinic crystal system, C2/m space group with one acetic acid molecule per formula unit (Z0 = 1/2) (see Figure 10a). The POM anion is positioned at the center of the crystallographic b-axis. O(4)-Mo(3)-O(7) axis of the [Mo6O19]2- cluster anion lies parallel to the 2-fold rotational axis of the monoclinic unit cell. The central atom of the POM anion, that is, O(4), is located in the inversion center (xyz = 1.0, 0.5, 1.0) and so it sits in a four-fold symmetrical site with quarter occupancy in the asymmetric unit. The Mo atoms of the POM are also found at two-fold symmetrical sites with half occupancies. NH4þ tetrahedron, too, has occupied a twofold symmetrical site with respect to the rotational axis.

Scheme 2. Stereoisomers of Dicyclohexyl-18-crown-6

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Figure 9. (a) ORTEP diagram of the compound 4 in 50% probability distribution. Only one crown ether cation has been displayed for a clearer view; (b) H-bonding interactions in the title compound displaying the geometry of the crown ether, bonds in bold = atoms upward, dashed bond = atoms downward, ax = axial and eq = equatorial. Atoms with additional label (#) are related to each other by the symmetry operations as mentioned in the footnote of Table 8; (c) ribbonlike arrangement of the supramolecular sandwiches through C-H 3 3 3 O interactions.

Thus, in the crystal structure of compound 5, there are more numbers of special position constraints compared to the last four crystal structures (compounds 1-4). Except C(7) and C(8) all other aliphatic carbon atoms of the crown ether DB30C10 experience higher thermal vibrations even at lower temperature (data have been collected at 100 K), which is very common for crystal structures associated with the structurally flexible crown ethers. In the crystal structure of compound 5, various bond lengths of the concerned crown ether are observed as follows: C(aryl)-C(aryl) 1.353-1.401 A˚, C(aryl)-O(aryl) 1.357-1.386 A˚, C(alkyl)-O(aryl) 1.431-1.441 A˚, C(alkyl)-O(alkyl) 1.3911.416 A˚, C(alkyl)-C(alkyl) 1.430-1.506 A˚ (see Table 6). Interestingly, 1:1 inclusion complexation between the ammonium ion and the crown ether has been observed in the crystal structure of the compound 5 where the guest cation is encapsulated in the crown ether cavity through four Nþ-H 3 3 3 O hydrogen-bonding interactions and the giant macrocycle wraps around the cation. In the last four crystal structures, it has been observed that the NH4þ cation has interacted with the crown ethers using three N-H bonds (Nþ-H 3 3 3 O hydrogen bonding interaction),

while the fourth N-H bond has been donated to an acceptor atom (bridging O atom of POM in case of 1, 2, 4 and solvent water molecule in case of 3). But, in the present case, as the cation has used all four N-H bonds to sit inside the crown ether cavity, Nþ-H 3 3 3 O interactions between the cation and the POM anion have been restricted. This kind of host-guest interaction is possible only if the host (here crown ether DB30C10) is highly structurally flexible. The C-H 3 3 3 O interactions between the crown ether and the POM anion play a significant role toward formation of one-dimensional supramolecular chainlike topology in the crystal lattice of compound 5 (see Figure 10b). The 2-fold rotational symmetry equivalence of the chain with respect to the crystallographic b-axis is clearly visible from the relevant picture. Alternately arranged crown ethers in between two POM anions are related to each other by the reflection and inversion symmetries. Apart from these intermolecular interactions between the crown ethers and the POM anions, the macrocycles are again stacked with each other by means of C-H 3 3 3 O and C-H 3 3 3 π interactions in the crystal lattice, giving rise to a complicated crystal packing (Figure 17 in the Supporting

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Table 6. Selected Bond Lengths for the Crystal Structure of the Compounds 4-5 Compound 4 C(1)-C(2) C(4)-C(5) C(7)-C(8) C(12)-C(13) C(15)-C(16) C(19)-C(20) C(7)-O(12) C(10)-O(14) C(17)-O(15) C(20)-O(11)

1.519(3) 1.525(4) 1.495(3) 1.525(4) 1.515(3) 1.496(3) 1.426(3) 1.422(3) 1.425(3) 1.431(3)

C(2)-C(3) C(5)-C(6) C(9)-C(10) C(13)-C(14) C(11)-C(16) C(1)-O(11) C(8)-O(13) C(11)-O(14) C(18)-O(16)

C(1)-C(2) C(4)-C(5) C(7)-C(8) C(13)-C(14) C(7)-O(10) C(10)-O(12) C(14)-O(9)

1.395(7) 1.401(7) 1.506(6) 1.488(10) 1.432(5) 1.390(7) 1.441(6)

C(2)-C(3) C(5)-C(6) C(9)-C(10) C(1)-O(9) C(8)-O(11) C(12)-O(13) C(15)-C(16)

1.526(4) 1.529(3) 1.502(4) 1.525(4) 1.520(3) 1.438(3) 1.429(3) 1.437(3) 1.424(3)

C(3)-C(4) C(1)-C(6) C(11)-C(12) C(14)-C(15) C(17)-C(18) C(6)-O(12) C(9)-O(13) C(16)-O(15) C(19)-O(16)

1.527(4) 1.523(3) 1.533(3) 1.532(4) 1.499(3) 1.434(3) 1.429(3) 1.441(3) 1.423(3)

1.379(10) 1.373(7) 1.474(10) 1.357(6) 1.412(6) 1.402(8) 1.64(3)

C(3)-C(4) C(1)-C(6) C(11)-C(12) C(6)-O(10) C(9)-O(11) C(13)-O(13) C(16)-O(14)

1.353(10) 1.398(7) 1.431(9) 1.386(5) 1.415(7) 1.403(8) 1.259(13)

Compound 5

Table 7. Hydrogen Bonding Parameters for the Compound 3a D-H 3 3 3 A d(D-H) d(H 3 3 3 A) C(8)-H(8A) 3 3 3 O(5)#3 0.97 2.66 C(12)-H(12) 3 3 3 O(10)#1 0.93 2.68 C(16)-H(16A) 3 3 3 O(3)#2 0.97 2.60 N1 3 3 3 O11 N1 3 3 3 O12 N1 3 3 3 O13 N2 3 3 3 O14 N2 3 3 3 O15 N2 3 3 3 O16 N1 3 3 3 O17 N2 3 3 3 O18 O(17) 3 3 3 O(14) O(18) 3 3 3 O(8)#4 O(17) 3 3 3 O(16)#5

d(D 3 3 3 A) — (DHA) 3.607(9) 166.2 3.525(13) 151.5 3.515(10) 156.6 3.013(6) 2.996(6) 2.978(5) 2.986(7) 2.977(6) 3.043(7) 2.798(8) 2.790(9) 3.059(8) 2.889(11) 3.118(9)

Table 9. Hydrogen Bonding Parameters for the Compound 5a D-H 3 3 3 A C(2)-H(2) 3 3 3 O(7)#1 C(10)-H(10B) 3 3 3 O(11)#2 C(13)-H(13A) 3 3 3 O(2) N(1)-H(1N) 3 3 3 O(10) N(1)-H(2N) 3 3 3 O(12)

d(D-H) d(H 3 3 3 A) d(D 3 3 3 A) — (DHA) 0.95 2.60 3.490(6) 155.2 0.99 2.56 3.463(8) 151.3 0.99 2.62 3.608(7) 176.6 0.80(6) 2.23(6) 3.000(6) 162(6) 0.83(6) 2.26(6) 3.043(5) 158(6)

a Symmetry transformations used to generate equivalent atoms: #1 -x þ 2, -y þ 1, -z þ 2; #2 -x þ 1/2, -y þ 3/2, -z þ 1.

a Symmetry transformations used to generate equivalent atoms: #1 x, y þ 1, z; #2 x, -y þ 1, z þ 1/2; #3 x, -y, z þ 1/2; #4 -x þ 1/2, y þ 1/2, -z þ 1.5; #5 -x, y, -z þ 1.5.

Table 8. Hydrogen Bonding Parameters for the Compound 4a D-H 3 3 3 A d(D-H) d(H 3 3 3 A) d(D 3 3 3 A) — (DHA) C(14)-H(14A) 3 3 3 O(5) 0.97 2.61 3.498 (3) 151.6 C(14)-H(14B) 3 3 3 O(9)#3 0.97 2.59 3.459(3) 149.0 C(17)-H(17A) 3 3 3 O(2)#2 0.97 2.51 3.352(3) 145.6 C(20)-H(20B) 3 3 3 O(6)#1 0.97 2.50 3.386(3) 151.6 N(1)-H(3N) 3 3 3 O(15) 0.92(4) 1.90(4) 2.814(3) 176(3) N(1)-H(4N) 3 3 3 O(13) 0.83(3) 2.11(4) 2.942(3) 178(3) N(1)-H(2N) 3 3 3 O(11) 0.86(4) 2.01(4) 2.848(3) 166(3) N(1)-H(1N) 3 3 3 O(4) 0.86(3) 2.03(3) 2.870(3) 166(3) a Symmetry transformations used to generate equivalent atoms: #1 x - 1, y, z; #2 x - 1/2, -y þ 1/2, z - 1/2; #3 -x þ 3/2, y þ 1/2, -z þ 3/2.

Information). C(10)-H(10B) 3 3 3 O(11) [3.463(8) A˚] interaction between two reflection equivalent crown ethers results in the formation of another one-dimensional chain propagating perpendicular to the crystallographic [011] plane (see Table 9 for the H-bonding parameters for compound 5). Crown ethers in this chain further interact with each other via C-H 3 3 3 π interactions as shown in Figure 11. Powder X-ray Diffraction Study. X-ray powder diffraction data for the grinded crystals of the compounds 1-4 have been recorded. An insufficient amount of sample has restricted us in doing so for compound 5. Similar diffraction patterns for the simulated data (calculated from the single crystal X-ray data) and observed data prove the bulk

Figure 10. (a) Crystal structure of the compound 5 in 30% thermal distributions. Only one inclusion complex has been shown for clarity; (b) one-dimensional chainlike topology of the supramolecular ensemble through C-H 3 3 3 O interactions viewed down the crystallographic [001] axis. H atoms and the acetic acid dimers have been excluded for clarity.

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Figure 11. Intermolecular C-H 3 3 3 O and C-H 3 3 3 π interactions between the crown ethers in the crystal lattice of the compound 5. Parameters for C-H 3 3 3 π interactions: d(C10-H10A) 0.99 A˚, d(H10A 3 3 3 Cg) 2.82(1)A˚, d(C10 3 3 3 Cg) 3.756(13)A˚, — (C10-H10A 3 3 3 Cg) 157.49(19) (symmetry: 0.5 - x, 1.5 - y, 1 - z) where Cg refers to the centroid of the concerned phenyl ring.

homogeneity of the crystalline solids (see Supporting Information for the PXRD patterns of the compounds 1-4).

spectroscopy has helped us to elucidate the cationic motifs in some of these compounds, that were not confirmed from their respective crystal structures.

Conclusion Although such crown-ether-cavity encapsulated ammonium complexes are long known, the same was not known with polyoxometalate association. We have described five ammonium-crown ether supamolecular complexes associated with polyoxometalate counteranions in compounds 1-5. We have shown the diversity in supramolecular interactions between the molecular fragments, as we analyze the supramolecular structures from compound 1 through compound 5. Changing the POM anion has not shown any disparity in the hydrogen bonding interactions between the ammonium ion and the POM anions as described in the crystal structures of compounds 1 and 2. But alteration of the crown ethers from less symmetrical to more highly symmetrical or from smaller to larger has shown diversity in crystal symmetry of the relevant compounds (1-5). All the crystal structures depict 1:1 supramolecular adduct formation between the hosts (crown ethers) and the guest (ammonium ion). In the crystal structures of compounds 1, 2, and 4, it has been observed that the ammonium cation interacts with the POM anions through the bridging O atom of the latter in all the cases. Possibly usage of the bridging O atom of the POM anion makes the latter in slightly tilted orientation (see Scheme 1) thereby bringing the terminal O atoms closer to the crown ethers to accept the C-H bond donation. This type of interaction might result in more close packing of the molecules in the crystal compared to the other mode of interaction, that is, interaction with the ammonium ion using the terminal O atom. In this mode of interaction, the bridging O atoms of the POM anion are apart from the crown ethers to accept the C-H bond donation from the latter (see Scheme 1). We also have performed detailed spectroscopic studies (UV-visible, IR, and NMR) to support the supramolecular structures in compounds 1-5 that are established by crystallography and purity of the solids have been checked by PXRD analysis. We have also shown how

Acknowledgment. The authors thank Department of Science and Technology (DST), India (Project No. SR/S1/IC-23/2007) and Centre for Nanotechnology at University of Hyderabad for funding. 400 MHz NMR and single crystal X-ray diffraction facility at University of Hyderabad by DST, Govt. of India is highly acknowledged. We thank Mr. Ganesh Kumar for his help while recording the PXRD data. T.C. and M.S. thank CSIR and UGC, India, respectively for their fellowships. Supporting Information Available: IR, 1H NMR spectra, complete list of distances and angles, X-ray crystallography related pictorial representations, simulated and experimentally observed PXRD patterns for the compounds 1-4 and X-ray crystallographic files in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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Supramolecular Architectures from Ammonium-Crown Ether Inclusion

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